(1) Field of the Invention
This invention relates to a switching power source device for supplying a desired AC/DC output by producing a pulse width-modulated waveform through ON-OFF control of switching elements serially connected to an input power source. The device eliminates high-frequency components of the pulse width-modulated waveform with a smoothing choke (hereinafter this operation will be referred to as "smoothing").
This invention particularly relates to a switching power source device which, in a system designed to control the output by alternatively switching a plurality of switching elements connected in pairs as in push-pull connection or bridge connection, precludes a current/voltage surge induced as a consequence of the on/off control of the switching elements. This device can be efficiently utilized in an AC uninterruptable power source system, a battery charging system, a controller drive system for motors, a DC constant voltage power source system, etc.
(2) Description of the Prior Art
The switching power source device has the virtues of small size and high efficiency, and therefore, finds extensive utility as a power source device in many fields such as data processing systems. The power source devices having a relatively large power capacity, and the AC power source devices designed to derive a sine-wave AC output from a DC input power source, are among the power source devices of this principle. For those power source devices which are adapted to effect control of output power by alternatively switching a plurality of switching elements connected in pairs, the push-pull connection or bridge connection are employed more often than not.
FIG. 3 is a schematic structural diagram illustrating a typical switching power source device of the conventional principle ("Power Semiconductor Circuits", pages 357-358, written by S. B. Dewan & A. Strengthen and published by John Wiley & Sons, 1975). FIG. 4 is a waveform diagram illustrating the operating principle of the power source device in FIG. 3.
A switch element 1 and filter means (composed of a choke coil 3 and a capacitor 4 which are connected in series with each other) are connected in series between the opposite terminals of a DC power source 5 which provides an output voltage with an unchanging, or constant, polarity as illustrated in FIG. 3. A load 6 is connected in parallel with capacitor 4. A diode D1 is connected in parallel with the series circuit of the choke coil 3 and the capacitor 4 in such a manner as to be reverse biased by power source 5. A junction node of the choke coil 3 and the capacitor 4 is connected to an output terminal 10.
During the operation, a rectangular wave voltage of modulated pulse width is generated at a node (junction point) 7 by alternately turning the switch element 1 on and off and regulating the time ratio of the alternate states.
When the time ratio of the ON-OFF states of the switch element 1 is controlled in the form of a sine wave as illustrated in FIG. 4 (a), for example, a rectangular wave voltage having a pulse width modulated as illustrated in the same figure is generated at the node 7 of the circuit of FIG. 3.
When this rectangular wave voltage has high-frequency components therein removed by means of the filter (LPF) composed of the choke coil 3 and the capacitor 4, a sine-wave pulsating output is produced at the output terminal 10 as illustrated in FIG. 4 (b) and is applied to the load 6.
A desired AC/DC output voltage is obtained by suitably varying the time ratio of the ON-OFF states of the switch element 1.
No problem would arise if the switch element 1 and the diode D1 were ideal switch elements and the signals for producing the alternate ON-OFF action of the switch were ideal rectangular waves. In the actual device, however, various problems occur whenever the switches are actuated because of the inherent characteristics of the switch elements. Now, these problems will be discussed below.
First, when the switch element 1 is on, an electric current flows to the load 6 through the circuit of power source 5, switch element 1 and choke coil 3. When the switch element is off, the load current continues to flow through the circuit of choke coil 3, load 6 and diode D1 because the load current flowing through the choke coil 3 does not immediately cease to exist.
When the switch element currently in the OFF-state is turned on again, the path for the electric current from the power source 5 would be switched safely to the choke coil 3 side if the diode D1 were instantaneously turned off by the reverse bias produced by the power source 5.
Actually, owing to the electric charges stored in the diode D1, however, the diode D1 is not instantaneously turned off by the application of the reverse bias and, as the result, the electric current flows for a brief period in the opposite direction. The brief period is called a storage time.
During the storage time, a surge current flows through the circuit of power source 5, switch element 1, diode D1 and back to power source 5 because the diode D1 is in a short-circuited state. The surge current causes such components as the switch element 1 to suffer power loss. The surge current renders it difficult to increase the switching frequency because such drawbacks as power loss, accumulation of heat in the switch element, and noise which are caused by the surge current are aggravated in proportion as the switching frequency is increased. Moreover, there is the possibility that the switch element will be destroyed when the surge current has an unduly large peak value.
It has been customary in the art to have such components as a reactor or a saturable reactor 81 connected in series to the switch element 1 and the diode D1 as illustrated in FIG. 5 (a) for the purpose of protecting the switch elements against the surge current and preventing the surge current from giving rise to current/voltage noise.
In this case, however, there arises the possibility that the energy stored in the reactor or the saturable reactor 81 during the storage time will give rise to a voltage surge during an interruption in the flow of electric current.
For the solution of said problem, it has been proposed to preclude the occurrence of the surge by connecting in parallel to the reactor 81, for example, a snubber circuit 80 composed of a resistor element and a diode as illustrated in FIG. 5 (b).
Even by this method, the surge cannot be prevented completely. Further, the charges stored in the reactor 81 are eventually consumed in the resistance of the snubber circuit 80. This method, therefore, fails to solve the problem of the aggravation of power loss and heat accumulation due to an increase in the switching frequency.